Eugenio Zallo1 Stefano Cecchi1 Valeria Bragaglia1 Fabrizio Arciprete1 2 Raffaella Calarco1

1, Paul-Drude-Institut für Festkörperelektronik, Berlin, , Germany
2, University of Rome Tor Vergata, Rome, , Italy

GeSbTe (GST) alloys lying along the GeTe/Sb2Te3 pseudobinary line exhibit impressive performance as non-volatile memory [1]. Epitaxial GST growth by molecular beam epitaxy (MBE) promoted the understanding of the structural properties, due to the superior thickness and interface quality control. In fact, along with the metastable cubic and stable trigonal crystalline phases, a cubic ordered GST has been recently demonstrated by gradually aligning vacancies in the growth direction [2]. The material is deposited in the stable phase by means of van der Waals (vdW) epitaxy, where the layers are weakly bonded to the substrate. This offers a toolbox for the investigation of 2D structures and the role of vdW gaps, but the possibility of functionalizing vdW bonded layers is still unclear.
In this talk I will first review the progress we made in the MBE growth of crystalline GST, which is dictated by the interplay between composition, phase and ordering. These parameters are correlated by a combination of X-ray diffraction and Raman spectroscopy techniques resulting in a growth phase diagram of Te flux as a function of substrate temperature [3]. I will then report on the usage of vicinal surfaces to induce strain at the step edges allowing for a combination of classical epitaxy and vdW epitaxy in vdW-bonded GST [4]. GexSb2Te(3+x) with almost GST124, trigonal phase and improved vacancy layers ordering is obtained for substrate offcut angles between 3-6°. Interestingly, the tilt of the epitaxial layer with respect to the Si substrate crystal axis is well reproduced by the Nagai model, which applies to covalently bonded materials. Finally, I will show Raman investigation of ultrathin GST films and GeTe/Sb2Te3 based superlattices.

[1] S. Raoux et al., Chem. Rev., 110, 240 (2010)
[2] V. Bragaglia et al., Sci. Rep., 6, 23843 (2016)
[3] V. Bragaglia et al., unpublished
[4] E. Zallo et al., Sci. Rep., 7, 1466 (2017)